a. Field of the invention
The present invention relates to a planar light emitting device or a planar illuminating device used for illuminating an advertisement, a signboard, a billboard, a guideboard, or the like. More particularly, the invention relates to a planar light emitting device having a uniform illumination over the entire light emitting device.
b. Description of the Related Art
Recent light emitting devices require a light emitting board having a thin thickness, and various kinds of illumination sources such as fluorescent lamps, light emitting diodes, incandescent lamps or the like are used to illuminate the light emitting devices. In many cases, the light sources are mounted on peripheral portions of the light emitting board. There is a strong need to illuminate the light emitting surface in a uniform manner. More specifically, light is introduced in a direction perpendicular to the light emitting surface from the peripheral portion of the light emitting board to thereby illuminate the light emitting surface. Since the light is introduced into the light emitting surface in a direction perpendicular to the light emitting surface from the peripheral portion of the light emitting board, it is very difficult to uniformly illuminate the light emitting surface due to the types of the light sources used for illumination and due to the number and the mounting position of the illumination tools.
The present inventor filed U.S. patent application Ser. No. 07/541,944 (now U.S. Pat. No. 5,057,974) to propose one countermeasure to this problem. According to the proposed method of that application, a reflecting member composed of a large number of reflecting units in the form of dots or lines with a density inversely proportional to a square of the distance as measured from each light source is provided under a lower surface of the light emitting surface, whereby the light reflected from the opposite surface is reflected by the reflecting units to uniformly illuminate the light emitting surface. The proposed method is suitable for a simple case where light sources for uniformly illuminating the object are used; however, there are still other difficulties involved in uniformly illuminating the object. In various cases, it is very difficult or sometimes impossible to provide dots or parallel lines with a density which is inversely proportional to the square of the distance from each light source due to the fact that the light sources are locally positioned at a peripheral portion of the light emitting surface and due to a deformed transparent substrate, the number, the type and the mounting position of the light sources. Also, even if the dots or parallel lines having a density inversely proportional to a square of the distance from the light sources are made, it sometimes would be impossible to illuminate the light emitting surface in a uniform fashion. For example, as shown in FIG. 3, when a linear, straight fluorescent lamp is provided on one side surface of the light emitting surface, the illumination of the central part of the fluorescent lamp is high, whereas the illumination of the end parts of the fluorescent lamp is low. Accordingly, it is difficult or impossible to illuminate the light emitting surface in a uniform fashion only by providing light reflecting surfaces composed of a large number of dots or lines having a distribution density inversely proportional to the square of the distance from each light source.
In order to solve this problem, the present inventor filed U.S. patent application Ser. No. 07/519,173 (now U.S. Pat. No. 5,138,782) and U.S. patent application Ser. no. 07/810,421. These applications are related to an improvement of the uniform illumination by providing a thin diffusion plate on the top surface of the light emitting surface. These methods would not be effective for the specific condition, since these methods still suffer from the problem that due to the shape of the light emitting surface of the light emitting device, the type and position of the light sources, or the irregularly reflected light from the reflection surface of the side walls of the light emitting device, a bright portion is generated in the light emitting surface even in a position remote from the light sources, whereas a dark portion is generated even at a position close to the light sources, resulting in the formation of illumination spots. Thus, even with the methods proposed in the applications, it would be difficult to obtain uniform illumination over the light emitting surface, and it would be safe to say that these methods would not bring about a satisfactory result.